Vladimir Putin said Thursday he does not rule out Russia sending arms to isolated North Korea in what Washington called an “incredibly concerning” statement that threatens instability on the divided Korean peninsula.

On a trip to Vietnam following a state visit to Pyongyang where Russia and North Korea signed a mutual defense pact, Putin also warned South Korea not to arm Ukraine.

“Those who send these (missiles to Ukraine), think that they are not fighting us, but I said, including in Pyongyang, that we then reserve the right to supply weapons to other regions of the world, with regard to our agreements with the DPRK,” Putin said, using the acronym of North Korea’s official name.

“I do not rule this one out.”

South Korea on Thursday called Russia’s treaty with the North a “grave concern,” with a senior official saying Seoul would “reconsider” its policy of not sending arms directly to Ukraine.

Putin brushed off the response, saying “South Korea has nothing to worry about.”

But he warned Seoul not to supply weapons to Kyiv, saying: “This would be a very big mistake.”

The United States expressed deep unease over Putin’s threat to send weapons to Pyongyang, warning such action could destabilize the Korean peninsula.

“It is incredibly concerning,” State Department spokesperson Matthew Miller told reporters.

“It would destabilize the Korean peninsula, potentially, depending on the type of weapons, and might violate UN Security Council resolutions that Russia itself has supported.”

Making his first visit to isolated North Korea in 24 years on Wednesday, Putin signed a strategic treaty with leader Kim Jong Un that included a commitment to come to each other’s aid if attacked.

Washington and its allies accuse North Korea of supplying ammunition and missiles to Russia for its war, and the new treaty has fueled fears of more deliveries.

The two countries have been allies since North Korea’s founding after World War II and have drawn even closer since Russia’s invasion of Ukraine in 2022 isolated Putin on the global stage.